1. Field of the Invention
The present invention relates to a pressure measurement sheet and a method of measuring pressure using the pressure measurement sheet. More specifically, the present invention relates to a pressure measurement sheet comprising microcapsules having a volume average particle diameter and a number average wall thickness in a specified relationship with each other.
2. Description of the Prior Art
Recently, it has become important to physically measure various planar pressures and linear pressures such as the clamping pressure of bolts in machinery, the pressure of automotive brakes, the pressure in molds, mold clamping pressures, bonding pressures between mold walls and a resin in the mold or the molding pressure within a mold in thermoplastic or thermo-setting molding processes, the contacting pressure of springs, the planar clamping pressures of carburetor joints, fuel pipes, fuel tanks, etc., in a fuel system, the pressure generated during collisions between a human body and other objects in an automobile, linear and planar pressures of rubber and plastic rolls, the effective limit and the effectiveness of gasket seals, hydraulic pressure in brakes, the pressure of gear teeth contact, the planar contact pressure between solids, the contact pressure of a cock in a cock valve, the contact pressure between curved surfaces, the contact pressure of rollers used in steel sheet material fabrication, the pressure between the sole of a persons's foot and the floor, the pressure between a human body and a chair or the like. However, methods of easy measurement of the above-described types of pressure are scarce, and most existing methods require complicated operations.
Conventional methods for measuring planar pressures or linear pressures and the like include a strain gauge method which utilizes the relationship between a stress and the strain caused thereby, a load meter method and using pressure-sensitive paint (strain-sensitive lacquer) and the like.
The employment of a strain gauge for the measurement of planar pressure, linear pressure or the like, however, not only requires large scale facilities including amplifiers, detectors, recorders and other devices, but also requires highly skilled operators for the devices and intricate calculations on the strength of material. This method has the further disadvantage that the surface of the materials to be measured must be flat and smooth, due to the inherent nature of the strain gauge. Strain gauges are described in detail by J. Yarnell in Strain Gauges, (published by Electronic Engineering Co., Ltd., London, 1957).
Using a pressure-sensitive paint (stress-sensitive lacquer) often results in unevenness in the paint coating, adhesion of the paint is often poor and complicated procedures are required.
The use of a load meter obstructs the miniaturization of the measurement system and, therefore, is not suitable for the measurement of planar or linear pressures.
There is also known a method of using a pressure-sensitive laminate (see U.S. Pat. No. 3,647,504) which is a mono-leaf recording paper comprising microcapsules of different wall thickness containing, respectively, various marking solutions of colors that differ depending on the wall thickness. In this pressure-sensitive laminate, different microcapsules are ruptured with different degrees of applied pressure to release marking solutions of different colors and, therefore, the applied pressure can be determined by checking the densities or hues of the colors released.
According to this method, however, pressure is shown only in an approximate order, and exact pressure values are difficult to determine. Further, in practical use, the pressure-sensitive laminate is defective as significant color staining of hands and clothing, etc., results.
In view of the foregoing, we performed a series of studies on methods of measuring pressure in all types of areas without large scale apparatus, the need for highly skillful operators or complicated calculations and, as a result, reached the method which comprises contacting a recording sheet with an object where pressure is to be measured to develop color under pressure and reading the pressure based on changes in the optical densities or hues of the colored images formed on the recording sheet, where the color formation results from pressurized contact between a microcapsule layer containing a color former and an adsorbent layer (see Japanese Patent Publication 33789/74).
However, a pressure sensitive recording paper or a pressure sensitive copying paper as disclosed in U.S. Pat. Nos. 2,712,507, 2,730,456, 2,730,457, 3,418,250 and 3,425,327 is defective when used as a measurement sheet as the pressure measurement is inaccurate in the range between about 1 kg/cm.sup.2 and about 200 kg/cm.sup.2. We further continued our studies to overcome these defects and, as the result, discovered a pressure measurement sheet which has an excellent accuracy for pressure measurement in the range of about 1 Kg/cm.sup.2 to about 200 Kg/cm.sup.2, thereby reaching the present invention.